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244                             LEAD AND  ITS ALLOYS

bottle holding about 200 c.c. and fitted with a ground stopper. After
addition of a further quantity of 10 c.c. of dilute sulphuric acid, the shaking
is continued for 10-15 minutes—until all the zinc is dissolved. The bottle
is emptied and washed into a half-litre measuring flask and made up to
volume. 100 c.c. of the solution are acidified with 5 c.c. of hydrochloric
acid (D = rio) and treated with 10 c.c. of 10% potassium iodide
solution, the iodine separated being titrated with sodium thiosulphate and
starch paste. The difference in c.c. between the volumes of dichromate
and thiosulphate, multiplied by 0-01635, giyes tne amount of zinc in 0-2
gram of the sample.

* *

Commercial zinc dust contains 80-90% Zn, 9-10% ZnO, 1-5-2% Pb, and
small quantities of copper, cadmium, arsenic, antimony, iron, sulphur, silica,
carbon, etc., and, sometimes, of calcium and magnesium oxides.


Lead is largely used as such and also enters into the composition of
numerous alloys.

Methods will be given here for the analysis of commercial lead and of
hard lead, alloys of lead with tin and antimony (solder, white antifriction
metal, white metal for fittings, etc.) being treated under tin and its alloys.


The analysis of commercial lead consists in determining the impurities
present (silver, copper, bismuth, cadmium, arsenic, antimony, iron, nickel,
cobalt, zinc and manganese). These impurities are, however, present in
very small proportions (refined lead contains, indeed, 99-96-99-99% Pb),
and it is therefore necessary to employ large quantities of the sample and
to conduct all the analytical operations with the greatest precision.

In a i-5-litre beaker, 200 grams of the metal are gently heated with a
mixture of 500 c.c. of nitric acid (D 1-2) and 500 c.c. of water. After stand-
ing for about 12 hours, the liquid is filtered, the insoluble residue (lead
antimonate) being collected on a small filter, washed, placed in a porcelain
crucible and kept apart (residue a).

The nitrate is treated with 62-63 c.c. of concentrated sulphuric acid
and left to cool and settle, the clear supernatant liquid being siphoned
into a 3-litre beaker. The residue is shaken with 200 c.c. of water acidified
with nitric acid, allowed to settle and the supernatant liquid added to the
first solution ; to ensure complete washing, this operation should be repeated
three or four times.

The liquid freed from lead sulphate is then rendered alkaline with ammo-
nia and treated with 25-50 c.c. of colourless ammonium sulphide. Together
with small quantities of lead which have passed into solution, the copper,
silver, bismuth, iron, etc., are precipitated as sulphides, whilst the arsenic
and antimony remain in solution in the alkaline sulphide. After remainingi : 3) are shaken for 5 minutes in a